A public address system (PA system) is an electronic sound amplification and distribution system with a microphone, amplifier and loudspeakers, used to allow a person to address a large public, for example for announcements of movements at large and noisy air and rail terminals or at a sports stadium. The term is also used for systems which may additionally have a mixing console, and amplifiers and loudspeakers suitable for music as well as speech, used to reinforce a sound source, such as recorded music or a person giving a speech or distributing the sound throughout a venue or building.

Simple PA systems are often used in small venues such as school auditoriums, churches, and small bars. PA systems with many speakers are widely used to make announcements in public, institutional and commercial buildings and locations. Intercom systems, installed in many buildings, have microphones in many rooms allowing the occupants to respond to announcements.

Sound reinforcement systems and PA systems may use some similar components, but with differing application, although the distinction between the two is not clear-cut. Sound reinforcement systems are for live music or performance, whereas PA systems are primarily for reproduction of speech.[1] In Britain any PA system is sometimes colloquially referred to as a Tannoy, after the company of that name now owned by TC Electronic Group, which supplied a great many of the PA systems used previously in Britain.[2]

In 1910, the Automatic Electric Company of Chicago, Illinois, already a major supplier of automatic telephone switchboards, announced it had developed a loudspeaker which it marketed under the name of the "Automatic Enunciator". Company president Joseph Harris foresaw multiple potential uses, and the original publicity stressed the value of the invention as a hotel public address system, allowing announcements to be heard in all the public rooms.[3] In June 1910, an initial "semi-public" demonstration was given to newspaper reporters at the Automatic Electric Company building, where a speaker's voice was transmitted to loudspeakers placed in a dozen locations "all over the building".[4]

A short time later the Automatic Enunciator Company was established in Chicago in order to market the new device, and a series of promotional installations followed.[5] In August 1912 a large outdoor installation was made at a water carnival held in Chicago by the Associated Yacht and Power Boat Clubs of America. Seventy-two loudspeakers were strung in pairs at forty-foot (12 meter) intervals along the docks, spanning a total of one-half mile (800 meters) of grandstands. The system was used to announce race reports and descriptions, carry a series of speeches about "The Chicago Plan", and provide music between races.[6]

In 1913, multiple units were installed throughout the Comiskey Park baseball stadium in Chicago, both to make announcements and to provide musical interludes,[7] with Charles A. Comiskey quoted as saying: "The day of the megaphone man has passed at our park." The company also set up an experimental service, called the Musolaphone, that was used to transmitted news and entertainment programming to home and business subscribers in south-side Chicago,[8] but this effort was short-lived. The company continued to market the enunciators for making announcements in establishments such as hospitals, department stores, factories, and railroad stations, although the Automatic Enunciator Company was dissolved in 1926.[5]

Early public-address system from around 1920 using a Magnavox speaker. The microphone (then called a "transmitter") had a metal reflector which concentrated the sound waves, allowing the speaker to stand back so it wouldn't obscure his face. The early vacuum tubes couldn't produce much gain, and even with six tubes the amplifier had an output power of only about 10 watts. To produce enough volume, the system used a horn loudspeaker. The cylindrical driver unit under the horn contained the diaphragm which was vibrated by the voice coil, and the sound waves produced were conducted to the open air through the flaring horn. The function of the horn was to couple the diaphragm more efficiently with the air, so horn speakers produced far more sound power from a given amplifier than a cone speaker. Horns were used in virtually all early PA systems, and in most systems today.

Edwin Jensen and Peter Pridham of Magnavox began experimenting with sound reproduction in the 1910s; working from a laboratory in Napa, California, they filed the first patent for a moving coil loudspeaker in 1911.[9] Four years later, in 1915, they built a dynamic loudspeaker with a 1-inch (2.5 cm) voice coil, a 3-inch (7.6 cm) corrugated diaphragm and a horn measuring 34 inches (86 cm) with a 22-inch (56 cm) aperture. The electromagnet created a flux field of approximately 11,000 G.[9]

Their first experiment used a carbon microphone. When the 12 V battery was connected to the system, they experienced one of the first examples of acoustic feedback.[9] They then placed the loudspeaker on the laboratory's roof, and claims say that the amplified human voice could be heard 1 mile (1.6 km) away.[9] Jensen and Pridham refined the system and connected a phonograph to the loudspeaker to be able to broadcast recorded music.[10] They did this on a number of occasions, including once at the Napa laboratory, at the Panama–Pacific International Exposition,[9] and on December 24, 1915 at San Francisco City Hall alongside MayorJames Rolph.[10] This demonstration was official presentation of the working system, and approximately 100,000 people gathered to hear Christmas music and speeches "with absolute distinctness".[9]

The first outside broadcast was made one week later, again supervised by Jensen and Pridham.[1][11] On December 30, when Governor of CaliforniaHiram Johnson was too ill to give a speech in person, loudspeakers were installed at the Civic Auditorium in San Francisco, connected to Johnson's house some miles away by cable and a microphone, from where he delivered his speech.[9] Jensen oversaw the governor using the microphone while Pridham operated the loudspeaker.

The following year, Jensen and Pridham applied for a patent for what they called their "Sound Magnifying Phonograph". Over the next two years they developed their first valve amplifier. In 1919 this was standardized as a 3-stage 25 watt amplifier.[9]

By the early 1920s, Marconi had established a department dedicated to public address and began producing loudspeakers and amplifiers to match a growing demand.[9] In 1925, George V used such a system at the British Empire Exhibition, addressing 90,000 via six long-range loudspeakers.[9] This public use of loudspeakers brought attention to the possibilities of such technology. The 1925 Royal Air Force Pageant at Hendon Aerodrome used a Marconi system to allow the announcer to address the crowds, as well as amplify the band.[9] In 1929, the Schneider Trophy race at Calshot Spit used a public address system that had 200 horns, weighing a total of 20 tons.[9]

The simplest PA systems consist of a microphone, an amplifier, and one or more loudspeakers. Simple and small PA systems of this type, often providing 50 to 200 watts of power, are often used in small venues such as school auditoriums, churches, and small bars. A sound source such as a Compact Disc player or radio may be connected to a PA system so that music can be played through the system.

Public address systems consist of input sources, amplifiers, control and monitoring equipment, and loudspeakers. The primary input sources are microphones for live announcements and a source of recorded sound. There may be a system which allows operators, or automated equipment, to select from a number of standard prerecorded messages. These input sources are fed into preamplifiers and signal routers that determine the zones to which the audio signal is fed. The preamplified signals are then passed into the amplifiers. Depending on local practices these amplifiers will usually amplify the audio signals to 50V, 70V or 100V speaker line level.[14] Control equipment monitors the amplifiers and speaker lines for faults before it reaches the loudspeakers. This control equipment is also used for separating zones in a PA system. The loudspeaker is used to convert electrical signals into sound.

Public Address System consisting of amplifiers, mixers, and routers for a major international airport

Some PA systems have speakers that cover an entire campus of a college or industrial site, or an entire outdoor complex (e.g., an athletic stadium). A large PA system may also be used as an alert system during an emergency.

Some analog or IP private branch exchange (PBX) telephone systems use a paging facility that acts as a liaison between the telephone and a PA amplifier. In other systems, paging equipment is not built into the telephone system. Instead the system includes a separate paging controller connected to a trunk port of the telephone system. The paging controller is accessed as either a designated directory number or central office line. In many modern systems, the paging function is integrated into the telephone system, and allows announcements to be played over the phone speakers.

Many retailers and offices choose to use the telephone system as the sole access point for the paging system, because the features are integrated. Many schools and other larger institutions are no longer using the large, bulky microphone PA systems and have switched to telephone system paging, as it can be accessed from many different points in the school.

PA over IP refers to PA paging and intercom systems that use an IP network instead of a centralized amplifier to distribute the audio signal to paging locations across a building or campus, or anywhere else in the reach of the IP network (including the Internet). Network-attached amplifiers and intercom units are used to provide the communication function. At the transmission end, a computer application transmits a digital audio stream via the local area network, using audio from the computer's sound card inputs or from stored audio recordings. At the receiving end, either specialized intercom modules (sometimes known as IP speakers) receive these network transmissions and reproduce the analog audio signal. These are small specialized network appliances addressable by an IP address just like any other computer on the network.[15]

Wireless Mobile Telephony (WMT) PA Systems refers to PA paging and [intercom] systems that use any form of Wireless Mobile Telephony System such as GSM networks instead of a centralized amplifier to distribute the audio signal to paging locations across a building or campus, or other location. The GSM mobile Networks are used to provide the communication function. At the transmission end, a PSTN Telephone, mobile phone, VOIP phone or any other communication device that can access and make audio calls to a GSM based mobile SIM card can communicate with it. At the receiving end, a GSM transceiver receives these network transmissions and reproduce the analogue audio signal via a Power Amplifier and speaker. The work on this was pioneered by Stephen Robert Pearson of Lancashire, England who successfully applied for and was granted patents for the systems which incorporate control functionality in addition to the audio announcement capabilities. The utilisation of the WMT (GSM) networks means that live announcements can be made from anywhere to anywhere in the world where there is WMT connectivity. The patents cover all forms of WMT ie 2G, 3G, 4G ..... xxG. A UK company called Remvox Ltd (REMote VOice eXperience) has been appointed under license to develop and manufacture products based upon this technology |url=http://www.remvox.co

London Underground employee making a Long Line Public Address system announcement using an RPA01 Radio Microphone at Bank Station

A Long-Line Public Address (LLPA) system is any public address system with a distributed architecture, normally across a wide geographic area. Systems of this type are commonly found in the rail, light rail and metro industries and allow announcements to be triggered from one or several locations to the rest of the network over low bandwidth legacy copper, normally PSTN lines using DSL modems, or media such as optical fiber, or GSM-R, or IP-based networks.[16]

Rail systems typically have an interface with a passenger information system (PIS) server, at each station linked to train describers which state the location of rolling stock on the network from sensors on trackside signaling equipment. The PIS system invokes a stored message to be played from a local or remote digital voice announcement system, or a series of message fragments to be assembled in the correct order, for example: " / the / 23.30 / First_Great_Western / Night_Riviera_sleeper_service / from / London_Paddington / to / Penzance / .... / will depart from platform / one / this train is formed of / 12_carriages /." Messages are routed via an IP network and are played on local amplification equipment. Taken together, the PA, routing, DVA, passenger displays and PIS interface are referred to as the customer information system (CIS), a term which itself is often used interchangeably with the term passenger information system.[citation needed]

Small clubs and bars use a fairly simple set-up, with large Front of House speakers and subwoofers aimed at the audience, and smaller monitor speakers aimed back at the performers so that they can hear their vocals and instruments. In many cases, the Front of House speakers are elevated, either by mounting them on poles or by "flying" them from anchors in the ceiling. The subwoofers do not need to be elevated. In the smallest coffeehouses and bars, the audio mixer may be onstage so that the performers can mix their own sound levels.[17] In larger bars, the audio mixer may be located in or behind the audience seating area, so that an audio engineer can listen to the mix and adjust the sound levels. The adjustments to the monitor speaker mix may be made by a single audio engineer using the main mixing board, or they may be made by a second audio engineer who uses a separate mixing board.

For popular music concerts, a more powerful and more complicated PA System is used to provide live sound reproduction. In a concert setting, there are typically two complete PA systems: the "main" system and the "monitor" system. Each system consists of a mixing board, sound processing equipment, amplifiers, and speakers. The microphones that are used to pick up vocals and amplifier sounds are routed through both the main and monitor systems. Audio engineers can set different sound levels for each microphone on the main and monitor systems. For example, a backup vocalist whose voice has a low sound level in the main mix may ask for a much louder sound level through her monitor speaker, so she can hear her singing.

The "main" system (also known as "Front of House", commonly abbreviated FOH), which provides the amplified sound for the audience, will typically use a number of powerful amplifiers driving a range of large, heavy-duty loudspeakers including low-frequency speaker cabinets called subwoofers, full-range speaker cabinets, and high-range horns. A large club may use amplifiers to provide 3000 to 5000 watts of power to the "main" speakers; an outdoor concert may use 10,000 or more watts.

The "monitor" system reproduces the sounds of the performance and directs them towards the onstage performers (typically using wedge-shaped monitor speaker cabinets), to help them to hear the instruments and vocals. In British English, the monitor system is referred to as the "foldback". The monitor system in a large club may provide 500 to 1000 watts of power to several foldback speakers; at an outdoor concert, there may be several thousand watts of power going to the monitor system.

At a concert in which live sound reproduction is being used, sound engineers and technicians control the mixing boards for the "main" and "monitor" systems, adjusting the tone, levels, and overall volume of the performance.

A line array speaker system and subwoofer cabinets at a live music concert

Touring productions will travel with relocatable large line-array PA systems, sometimes rented from an audio equipment hire company. The sound equipment moves from venue to venue along with various other equipment such as lighting and projection.

All PA systems have a potential for audio feedback, which occurs when sound from the speakers is picked up by the microphone and is then re-amplified and sent through the speakers again. It often sounds like a loud high-pitched squeal or screech, and can occur when the volume of the system is turned up too high. Feedback only occurs when the loop gain of the feedback loop is greater than one, so it can always be stopped by reducing the volume sufficiently. Sound engineers take several steps to maximize gain before feedback, including keeping microphones at a distance from speakers, ensuring that directional microphones are not pointed towards speakers, keeping the onstage volume levels down, and lowering gain levels at frequencies where the feedback is occurring, using a graphic equalizer, a parametric equalizer, or a notch filter.

^"Tannoy definition". Cambridge Online Dictionary. Retrieved 19 May 2015. a system of equipment that is used for making speech loud enough for a large number of people to hear, especially in order to give information